A fingerprint reader's basic job is to acquire an image of a fingertip so that it can be determined whether the pattern of ridges and valleys in the image matches a pattern in a database of stored images. The two most common technologies for imaging a fingerprint are optical scanning and capacitance scanning. Another more recently developed technology uses an ultrasonic sensor.
One common configuration of an optical fingerprint reader uses a right-angled prism, a light source, a lens group, and an image sensor, such as a CCD device. When a fingertip is on the contact surface (a flat side of the prism), its ridges are pressed onto the surface while its valleys are spaced from it. Light from the light source reaches the contact surface after passing through the prism. If the light reaches a valley, total internal reflection causes all the light to reach the image sensor after passing through the lenses. On the other hand, if the light reaches ridges pushed onto the surface, some light reaches to the image sensor and some light is absorbed in the ridges. Changes in luminous intensity between light reflected from valleys and light reflected from ridges are calculated by the image sensor to obtain the fingerprint image. In short, optical prism-based fingerprint readers rely on the fact that fingerprint ridges placed on the contact surface disrupt the prism's total internal reflection, enabling high contrast fingerprint imagery.
Prism-based fingerprint readers can use either a bulk-prism or a micro-prism sheet. Using a micro-prism sheet reduces the sensor's size, weight, and cost while increasing its robustness and manufacturability. An example of a fingerprint reader implemented with a micro-prism sheet is described in U.S. Pat. No. 5,448,649.